1996 Mercury Tracer car chugs and sputters

1996 Mercury Tracer mileage: 135,000. It all started with a simple occasional sputter at low speeds. Right away, we had it serviced, changed oil, oil filter, air filter, and PCV valve. This was to no avail. Then it turned to sputtering up to about 40 mph and then the occasional chug while driving. I thought I had gotten bad gas or sucked debris off the bottom of the gas tank. I had the fuel filter replaced and the fuel injection system cleaned. Things just got worse. I was told to replace the O2 sensor and I did, but the problem simply progressed. Now the car chugs and sputters all the way to 80 mph. When I accelerate from a stop, it is as if the car isn't receiving fuel at all, even if I floor it. After creeping through an intersection (or about ten seconds) I get this sudden burst of power as if it's saying, "Ah hah! This is where the food has been hiding! Now I'm going to eat it all!” I considered the spark plugs briefly but they are those copper Rapid Fire ones and I'm pretty sure I have a while before they need replacement. I don't know what else to do. This problem only presents itself when my foot is on the accelerator. My gas mileage has decreased seven to ten mpg. Could it be the fuel pump? I know there's a crack along my muffler pipe or exhaust pipe could the problem stem from this? PLEASE do let me know. Thanks so very much!